The present invention relates to a terminal device such as a telephone or facsimile for a network system in which an information providing service is available, and more particularly to a terminal device which is capable of receiving information provided through a telephone line during a silent period between ringing signals.
Conventionally, a caller-ID system is known as one of information providing services utilizing telephone lines. The caller-ID system is a system for providing certain signal representing information related to a caller during a silent period between intermittent ringing signals. The information signal is added by an exchanger (a switching device) of a telephone network system and transmitted to a terminal device at a receiving station.
The information carried by the caller-ID data may include a date of the call, the telephone number of the caller (calling station), the telephone number of the receiver (receiving station), the name of the caller, the condition of the network system and the like. Various information may be inserted as the caller-ID data in various countries and/or by various central offices which provide similar caller-ID services.
In order to receive the caller-ID data signal which is inserted during the silent period between the ringing signals, it is necessary to make a demodulation method of a modem to correspond to a modulation method for generating the caller-ID data to be received. Generally, an unmodulated mono-frequency signal is added immediately before the caller-ID data as a trigger signal. In the conventional terminal device which is designed to receive the caller-ID data, when the trigger signal, i.e., the unmodulated mono-frequency signal is detected, a data reception procedure of the caller-ID data is started.
In the conventional terminal device as described above, if the line condition is bad and the unmodulated mono-frequency signal may not be detected due to occurrence of noises, it becomes impossible to receive the caller-ID data.